The present disclosure relates to an image forming apparatus preferably applied in a copying machine, a printer or the like.
An image forming apparatus according to an electrophotographic technique carries out image forming process in procedures of developing an electrostatic latent image formed on a surface of a photosensitive drum by a toner, and transferring and fixing the developed toner image onto a recording medium.
Recently, as a manner electrically charging the photosensitive drum in such an image forming apparatus, for example, a contact charging manner arranging a roller type charger to come into contact with or to come into close contact with the photosensitive drum so as to electrically charge the photosensitive drum is widely applied. In general, in the surface of the photosensitive drum, an organic photoconductor (OPC) is applied and, in the charger, a conductive rubber roller is applied.
For example, there is an image forming apparatus including a contact type charger, a voltage applying means for the charger, a developing device, a transferring/separating device and a surface potential detecting means. The contact type charger electrically charges the surface of the photosensitive drum. The developing device develops the electrostatic latent image formed by an exposure device (an optical system) to the toner image. The transferring/separating device transfers the toner image onto a sheet. The surface potential detecting means measures a surface potential of the photosensitive drum. This image forming apparatus measures the surface potential of the photosensitive drum by surface potential detecting means and carries out feedback control of an applied voltage to the charger.
Incidentally, it is known that it is difficult to electrically charge the surface of the photosensitive drum composed of the OPC to a rated electrical potential in a short time by the contact charging type charger. The above-mentioned image forming apparatus generally carries out application of a charging bias (pre-charging process) in advance of a start of the actual image forming process. In the above-mentioned image forming apparatus, in order to shorten a time from a print instruction of a user to print completion (a first print time), in general, when the surface potential of the photosensitive drum is electrically charged to an electrical potential enabling the image forming by the pre-charging process, even if the surface potential of the photosensitive drum has not reached an objective surface potential, the image forming process is started with respect to a first sheet.
In a situation where the photosensitive drum has not reached the objective surface potential, in a case of starting the image forming process, an electrical potential difference between the surface potential and a developing bias in that time is smaller than a rated electrical potential difference. In this case, a phenomenon making the toner adhere to a white area on which the image is not formed onto the first sheet in a printing job, so called fogging, occurs.